Lipids and proteins associate covalently to form lipid-linked proteins and noncovalently to form lipoproteins. The lipid portions of lipid-linked proteins anchor their attached proteins to membranes and mediate protein-protein interactions. Proteins form covalent attachments to lipids in several ways, one of which is the covalent attachment of isoprenoid groups, mainly the C15 farnesyl and C20 geranylgeranyl residues.
In mammals, geranylgeranyltransferase is known to catalyze the transfer of a geranyl-geranyl moiety from geranylgeranyl pyrophsophate to both cysteines in Rab proteins (Farnsworth, C. C. et al. (1994) Proc Natl Acad Sci USA 91(25):11963-11967). Rab proteins are Ras-related small GTPases that are geranylgeranylated on cysteine residues located at or near their C termini. Farnesyltransferase catalyzes the addition of farnesyl groups to the C termini of protein such as Ras.
Mammalian protein geranylgeranyl transferases types 1 and 2 are heterodimers composed of an alpha and beta subunit. The alpha subunit shows homology to the alpha subunits of a closely related enzyme, farnesyltransferase. Farnesyltransferases have been described in pea, tomato, and Arabidopsis, but have not been described in monocots (Yang et al (1993) Plant Physiology 101:667-674). Plant farnesyltransferases also consist of alpha and beta subunits. The beta subunit is responsible for peptide-binding and contains a catalytic zinc ion. The beta subunit belongs to the protein prenyltransferase beta subunit family. The geranylgeranyl transferase beta subunit also belongs to the protein prenyltransferase beta subunit family. The beta subunits of the type 1 and 2 geranylgeranyltransferases have not been previously described in plants. Work done in yeast has established that protein geranylgeranyltransferases are distinct from the closely related protein farnesyltransferases.
It has been shown that defects in farnesyltransferase activity enhances plant hormone abscisic acid (ABA) levels. In a normal plant ABA levels increase in response to water deficits. An increase ABA in leaf tissue triggers the closure of leaf stomata to decrease water loss via transpiration (Pei et al. (1998) Science 282:287-290). Plants with a decrease in farnesyltransferase activity could confer enhanced tolerance to drought stress in plants. Thus, there is a great deal of interest in identifying the genes that encode farnesyltransferase in plants. These genes may be used in plant cells to control cell growth and produce plants with improved water stress tolerance. Accordingly, the availability of nucleic acid sequences encoding all or a portion of farnesyltransferase proteins would facilitate studies to better understand cell growth in plants, provide genetic tools to control cell growth and improve tolerance to drought in mature plants.